Recent evidence indicates that abnormal accumulation of multiple disease-associated proteins co-exists in several neurodegenerative disorders. For instance, Abeta, tau and TDP-43 pathology can be found in some Alzheimer's disease cases, while tau and TDP-43 abnormalities are evident in ALS with cognitive impairment. However, very little is known about how tau, Abeta and TDP-43 interact with each other in different cell types and in different disease context. Unfortunately, these questions are difficult to address because these proteins induce developmental abnormalities or early lethality when co-expressed in transgenic animals. Thus, a new strategy for the spatio-temporal control of multiple genes will be essential to address this challenge. To that end, we have developed a new genetically encoded light-switchable system based on the fast, reversible photoactivation of Phytochrome B (PhyB) from plants. Phytochromes are sensory photoreceptors that regulate plant growth in response to light signals and require a covalently linked chromophore for proper function. In response to red light, the phytochrome-chromophore complex changes its conformation and translocates to the nucleus to trigger signal transduction. In the dark or under far-red light, its conformation returns to the inactive state and PhyB accumulates in the cytosol. Thus, we will exploit this light-dependent, conformer-specific interaction in plants to develop a new light switchable gene expression system in transgenic animals. This system, which we have called PhotoGal4, will encode for all the elements required for the formation of the phytochrome-chromophore complexes in animal cells along with protein motifs required for transcriptional activity. We will use the fruit fly Drosophila melanogaster as our initial animal model to implement this new optogenetic system. In Specific Aim 1, we will characterize the light- dependent activation and reversibility of PhotoGal4 in adult flies. In Specific Aim 2, we wil use PhotoGal4 to generate optogenetic models of primary and secondary CNS proteinopathies involving Abeta, tau and TDP-43. Our central hypothesis is that PhotoGal4 will bypass lethality issues by controlling gene expression in response to light quantity, duration and direction. This proposal is significant, innovative and potentially transformative because a successful implementation of PhotoGal4 will provide a tool for directing expression of any gene with unprecedented precision. In addition, it will provide a unique opportunity to define the sequence of events that orchestrate concomitant pathology associated with Abeta, Tau and TDP43 interactions. The elucidation of how these three proteins interact in vivo will be a significant stp forward to define the mechanisms underlying some of the most important primary and secondary CNS proteinopathies.